The following abbreviations are herewith defined, at least some of which are referred to within the following description about at least the prior art and/or the present invention.
ALMSD Advanced Legacy Mobile Station Domain
BS Base Station
CDMA Code Division Multiple Access
FACDIR2 Facility Directive
IOS Inter-Operability Specification
IP Internet Protocol
LMSD Legacy MS Domain
MGW Media Gateway
MS Mobile Station
MSC Mobile Switching Center
MSCe Mobile Switching Center emulation
PSTN Public Switched Telephone Network
RTP Real-time Transport Protocol
SDP Session Description Protocol
SIP Session Initiation Protocol
SS7 Signaling System 7
CDMA voice services (e.g., call origination or call termination) are supported by establishing a dedicated signaling and bearer connection between a Mobile Station (MS), a Base Station (BS), and a Mobile Switching Center (MSC). The BS controls the air interface resources and the MSC performs call control for the voice services provided to the MS. If the MS is moving the signal strength between the MS and the BS might decrease to a level such that a different BS might be better able (have a higher signal strength) to establish a dedicated signaling and bearer connection to the MS. A handoff, or handover, occurs when the air interface resources supporting an ongoing voice service is transferred from an anchor BS (the BS initiating the handoff) to a target BS (the BS receiving the handoff request). An intra-MSC handoff occurs when both the Anchor BS and the Target BS are served by the same MSC. An inter-MSC handoff occurs when the anchor BS and the target BS are served by different MSCs. During an inter-MSC handoff (also referred to as intersystem hard handoff or intersystem handoff) the MSC initiating the handoff is called the Anchor MSC and the MSC receiving the handoff request is called the Target MSC.
A MSCe (Mobile Switching Center emulation) is a network entity originally defined for Legacy MS Domain (LMSD) support. The MSCe provides signaling capabilities comparable to those of a legacy MSC but has only bearer management capabilities. Some of the MSCe functionality includes:                the establishment, management, and release of voice calls and bearer resources associate with a voice call (for example, the use of Session Initiation Protocol (SIP) signaling for call control and the use of H.248 signaling to control bearer resource allocation);        call modifications for ongoing voice calls (for example, call hold, the addition of a third party to the call, the redirection of the call to a different party); and        interworking between the TIA/EIA-41 signaling protocol and the SIP signaling protocols.        
For intersystem handoff communications between an Anchor MSCe and a Target MSCe that both support Legacy Mobile Station Domain (LMSD) the TIA/EIA-41 signaling protocol and the SIP signaling protocol are required. For the sake of convenience an intersystem handoff between an Anchor MSCe and a Target MSCe that both support LMSD is referred to herein as an “LMSD handoff”. Details of what is required for an MSCe to support LMSD are defined in the LMSD standard entitled “Legacy MS Domain Step 2” Version 1.0, dated February, 2006.
For intersystem handoff communications between an Anchor MSCe and a Target MSCe that both support Advanced Legacy Mobile Station Domain (ALMSD) only the SIP signaling protocol is required. For the sake of convenience an intersystem handoff between an Anchor MSCe and a Target MSCe that both support ALMSD is referred to herein as an “ALMSD handoff”. If the Anchor MSCe and a Target MSCe both support ALMSD no Signaling System 7 (SS7) connectivity is required between the two MSCes. Details of what is required for an MSCe to support ALMSD for the purpose of an intersystem handoff, with the Anchor MGW established in the voice bearer path after the intersystem handoff, are given in this invention.
The MSCe controls bearer resources using International Telecommunications Union Telecommunication (ITU-T) Recommendation H.248 signaling to a Media Gateway (MGW). The MGW has the ability to connect to the IP-based core network as well as to the circuit-based Public Switched Telephone Network (PSTN). The MGW may provide vocoding and/or transcoding functions to the bearer traffic. The resources provided by the MGW, including transcoding resources, can be used to support bearer channels that are contained entirely within the IP environment.
Although an LMSD handoff (i.e., as defined in Legacy MS Domain Step 2″ Version 1.0, dated February, 2006) works well it still has problems that can occur after a MS has successfully established a connection with the Target BS. These problems are discussed below.
In one problem, a LMSD handoff does not take into consideration the use case (scenario) of the MS successfully establishing a connection with the Target BS yet resulting in the Anchor MGW being unable to support (e.g., due to operator policy, or Anchor MGW inability) transcoding between the codec established for the B-Party to Anchor MGW bearer path and the codec established for the Anchor MGW and the MS connected to the Target BS.
In another problem, a LMSD handoff does not take into consideration the use case (scenario) of the MS successfully establishing a connection with the Target BS yet resulting in the Anchor MGW selecting a less optimal codec (due to for example voice quality, bandwidth, or operator policy) for the voice bearer path.
In yet another problem, the Anchor MSCe limits the Target network choice of codecs for establishing the voice bearer path back to the Anchor network. In the prior art the Anchor network only offered the Target network the pre-handoff active codec. In view of the foregoing, it can be seen that there has been and is a need to address these problems and other problems associated with an existing LMSD handoff.